The present invention relates to a nozzle protection cap for an arc plasma torch. The arc plasma torch can be used both for dry cutting and underwater cutting of different metal workpieces.
During plasma cutting, an arc (pilot arc) is first ignited between a cathode (electrode) and anode (nozzle) and then directly transferred to a workpiece in order to carry out cutting.
The arc produces a plasma which is a highly heated, electrically conductive gas (plasma gas) consisting of positive and negative ions, electrons as well as excited and neutral atoms and molecules. By way of plasma gas, gases such as argon, hydrogen, nitrogen, oxygen or air are used. These gases are ionised and disassociated through the energy of the arc. The plasma beam produced is used to cut the workpiece.
A modern arc plasma torch consists largely of base components such as a torch body, electrode (cathode), nozzle, one or a plurality of caps such as the nozzle cap and nozzle protection cap, which surround the nozzle, and connections which are used to supply the arc plasma torch with power, gases and/or liquids. Nozzle protection caps can be used to protect the nozzle during the cutting process against the heat and sprayed-out molten metal of the workpiece.
The nozzle can consist of one or more components. With directly water-cooled arc plasma torches the nozzle is held by a nozzle cap. Cooling water flows between the nozzle and the nozzle cap. A secondary gas then flows between the nozzle cap and nozzle protection cap. This serves for the creation of a defined atmosphere, for tapering the plasma beam, and for protection against spraying during penetration.
In the case of gas-cooled arc plasma torches and indirectly water-cooled arc plasma torches, the nozzle cap can be omitted. The secondary gas then flows between the nozzle and nozzle protection cap.
The electrode and the nozzle are arranged relative to each other in a certain spatial relationship and define a space, the plasma chamber, in which the plasma beam is produced. The plasma beam can be greatly influenced in its parameters, such as, for example, diameter, temperature, energy density and through-flow rate of the plasma gas, through the design of the nozzle and electrode.
Electrodes and nozzles are produced from different materials and in different forms for different plasma gases. They are generally produced from copper and directly or indirectly water-cooled. Depending upon the cutting task and electric power of the arc plasma torch, nozzles are used which have different inner contours and openings with different diameters and thus provide optimum cutting results.
For example German Document DE 10 2004 049 445 A1 shows an arc plasma torch with a water-cooled electrode and nozzle and a gas-cooled nozzle protection cap. The secondary gas is fed through a nozzle protection cap holder inside past a screw connection region between the nozzle protection cap holder and a nozzle protection cap through a secondary gas channel formed between the nozzle protection cap and a nozzle cap to a plasma beam.
European Document EP 0 573 653 B1 relates to an arc plasma torch with a water-cooled electrode and nozzle and also a water-cooled nozzle protection cap. As in the case of the arc plasma torch disclosed in DE 10 2004 049 445 A1, in EP 0 573 653 B1 a secondary gas is fed within a nozzle protection cap holder inside past a screw connection region between the nozzle protection cap holder and a nozzle protection cap to a plasma beam. Also as in the arc plasma torch disclosed in DE 10 2004 049 445 A1, the arc plasma torch of EP 0 573 653 B1 comprises insufficient cooling of the nozzle protection cap for certain applications.
In addition, the arc plasma torch of EP 0 573 653 B1 is designed so that an annular cooling water chamber is formed within the base end region of the nozzle protection cap. Flowing cooling water cools the nozzle protection cap. This structure has the additional disadvantage that upon unscrewing the nozzle protection cap, the cooling water leaves the cooling chamber and drips or runs on to the outer surface of the nozzle cap and the inner surface of the nozzle protection cap. This gives rise to cooling medium residue in the secondary gas chamber formed by the nozzle cap and the nozzle protection cap, which both impairs cutting quality and operational security and also leads to loss of cooling medium.